A hot jet, cold jet and a loop modulation system in a two-dimensional gas chromatograph are disclosed in U.S. Pat. No. 7,258,726 to Ledford. Ledford discloses a modulation tube having a loop modulator structure. Two portions of the modulation tube can be thermally modulated simultaneously by a thermal modulation device which includes a cold jet assembly including a cold jet nozzle, and a hot jet assembly including a hot jet nozzle to modulate the temperature of portions of the modulation tube. This modulation system, which requires one set (cold and hot) jets to achieve the modulation process is an improved design from an older dual-Jet modulation system.
The loop modulator employs hot and cold jets of gas to effect two-stage thermal modulation. The two stages are formed by looping a segment of capillary tubing through the path of a single cold jet. The tubing between the two cold spots thus formed comprises a delay loop. An example of the mechanical assembly of the loop modulator is illustrated in FIG. 1. The cold jet subassembly 1 consists of a steel tube of approximately 3.0 mm inside diameter housed in a steel outer tube of some 19 mm outside diameter. The space between the inner and outer tubes is evacuated with a mechanical roughing pump and valved off, thereby forming a vacuum insulated housing around the cold jet. Vacuum insulation is essential for the introduction to the GC oven of a cryogenically cooled gas stream. A hot jet subassembly 2 is mounted at right angles to the cold jet by means of machined brackets, which also provide means of holding the modulator tube in the paths of the gas jets. The loop modulator tube 3 is housed in a folded metal holder 4 studded with machined “buttons” which mount in slots milled into the bracket structure. The loop modulator tube is held in place within the modulator holder by a folded piece of Kapton (Dupont) film 5, which functions as a spring tensioner. The modulator tube is installed and uninstalled in a matter of seconds by sliding it into or out of the slotted bracket structure 4.
In order for this loop modulator to work in the most efficient mode, it is required that the cold jet 1, hot jet 2 and the loop modulator tube 3 be aligned in a single point. At that specific point, the cold jet has the most cooling power to trap eluate through the looped modulator tube and the hot jet is the best focused to blow hot gas to release the trapped material in the looped modulator tube. The alignment scheme is shown in FIG. 1.
Since the cold jet subassembly consists of a steel tube of approximately 3.0 mm inside diameter, cold jet flow through the tube will further spread out due to free expansion when it passes into the open space below the cold jet. The length/area of column being cooled by this cold jet is slightly longer/larger than 3.0 mm with an uneven cooling efficiency through the entire cooled modulator tube covered by this cold jet. In the center of the cold jet, the column temperature will be the lowest and the modulator tube temperature will be higher at the edge of the cold jet coverage. The bottom view of the modulation tube is shown in the FIG. 2. The bottom view shows the loop modulator tube 3, cold jet 1, and hot jet 2.
This uneven modulator tube temperature and the length of the modulator tube being cooled by the cold jet will directly affect the trapping efficiency during the modulation in a comprehensive two-dimensional gas chromatography procedure. If there is a slight misalignment or the alignment has been changed during the experiment, the cold jet trapping process will become imperfect. However, the most significant impact on this imperfect trapping efficiency is the hot jet release process. In most cases, the hot jet either does not have enough thermal energy to completely release the eluent being trapped or does not spread wide enough to cover the entire length cooled by the cold jet. This will create a cold spot in the looped component that will cause the modulation operation to completely fail or cause every separated component to carry a long tail in the second dimensional separation. Both cold spots and tailing peaks will cause the failure of the comprehensive two-dimensional gas chromatography separation. However, the prior art modulator requires that the cold jet nozzle, hot jet nozzle, and loop modulator be properly aligned in order to reach the best efficiency of the modulation. Three point or three dimensional alignment of the hot jet, cold jet and modulation tube are required. If there is any one component misaligned, there will be a very significant impact on the modulation process and result in a failure of the comprehensive two-dimensional separation and any experimental results obtained therefrom.
There is a need for an improved modulator design for a comprehensive two-dimensional gas chromatography having improved alignment capabilities to enhance reliability.